Wavelength division multiplexing (WDM) is a method used to transport several independent optical signals on one optical fiber. WDM can be used for point-to-point communication where two nodes, one in each end of the fiber, are connected, or for multipoint-to-multipoint communication where several nodes share the transmission capacity on one fiber or on a pair of fibers. In the case of multipoint-to-multipoint communication the fiber or fibers used usually forms a closed loop, a ring, in order to have two different routes between a pair of nodes in the case of fiber or other equipment failure. Bus networks in which the end nodes are located at different places are also possible. An example of an add-drop node in such a ring or bus WDM network is given in the published European patent application 0905936.
A schematic of a WDM ring network is shown in FIG. 1. The traffic in the two parallel ring fibers or fiber paths 1a, 1b goes in opposite directions and each fiber path carries traffic in one direction only. In the nodes 3 wavelength selective optical add and drop (“add/drop”) filters, not shown, are provided that allow a selected wavelength channel to be added/dropped to/from a ring fiber with a low loss, and let all other wavelength channels pass along the ring fiber, through the node, with a low loss. In FIG. 1 the arrows 41, 42 in and out of the nodes symbolize interfaces for the payload portion of the information transported in the ring network.
In FIG. 2 is illustrated how bidirectional communication between a pair of nodes, nodes A and B, is achieved using unidirectional communication on each of two transmission fibers 1a, 1b. Usually the same wavelengths are used for the light signals on the two fibers. Totally four add or drop filters connected in the transmission fibers 1a, 1b are needed. It has also been proposed to use bidirectional communication using a single optical transmission fiber. Such communication in a single fiber ring network is disclosed in the article by Hitashi Obara, Hiroji Masuda and Kazuo Aida, “Transmission over a 200-km single-fiber bidirectional ring network with reconfigurable WDM add/drop repeaters”, 11th International Conference on Integrated Optics and Optical Fibre Communications 23 European Conference on Optical Communications IOOC-ECOC97, Edinburgh, UK, Sep. 22-25, 1997, pp. 9-12. Bidirectional add/drop multiplexers B-ADMs are used that each contain two four-port circulators directly connected in series with each other. One circulator transmits from the B-ADM in a first direction, e.g. to the left, and receives from the same direction, e.g. from the left, and the other circulator transmits from the B-ADM in a second direction, e.g. to the right, and receives from the same direction, e.g. from the right. Two branches of the circulators are through filters connected to an add port and a drop port, respectively, of the B-ADM.
Furthermore, in the published European patent application No. 0 729 248 for AT&T, inventor Giles, bidirectional communication over a single fiber between two terminals is disclosed. Channels can be added and dropped in bidirectional amplifiers or in add/drop multiplexers. In such a bidirectional amplifier two four-port circulators are used that each have a first port connected in the single fiber path, second and third ports connected to each other through conventional optical fiber amplifiers and a fourth port connected to add and drop ports through an optical power coupler. In the branch to the coupler gratings are connected. An add/drop multiplexer comprises two six-port circulators interconnected by fiber paths comprising filters at two of their ports.